When two letters fall together, the usual assumption is that a distinction was lost. That is only sometimes true, and this letter is a good case for taking the assumption apart.
In Russian the letter went and the distinction stayed. The sequence of glide plus e is still pronounced, still distinct from plain e, and still written, but by a rule rather than by a letter: the reader derives it from the position of the vowel letter and from what stands before it. The information is fully recoverable, and no Russian speaker is ever in doubt. What was lost is a one to one relation between sign and sound, which is a loss of transparency rather than of content.
In Ukrainian the letter went and a different letter took the job, so the distinction stayed both in speech and on the page.
In the earliest Slavonic manuscripts there is a third situation, which is scribes writing the two letters interchangeably before either language had settled anything. That variation is often read as carelessness. It is at least as likely to be evidence that the scribe's own speech no longer supported the distinction the exemplar in front of him was making, which makes it data rather than error.
The general lesson applies well beyond this alphabet. A letter disappearing tells you about the writing system. To learn what happened to the language you have to look at what the surviving letters were made to do afterwards, and here they were made to do more.